In an image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes an optical beam scanning apparatus that forms an electrostatic latent image on a photoconductive drum by irradiating a laser beam (a luminous flux) on the surface of a photoconductive drum and scanning the laser beam.
Recently, besides a monochrome machine including a scanning optical system that uses a single light source, a tandem color machine is proposed. For the tandem color machine, a method of increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) in one laser unit (a multi-beam method) is proposed for the purpose of realizing an increase in speed of scanning on the surface of a photoconductive drum. In the multi-beam method, plural beams for each of color components (e.g., yellow, magenta, cyan, and black) emitted from the respective light sources are subjected to processing in a pre-deflection optical system and are changed to one beam and made incident on a polygon mirror. The beam deflected by the polygon mirror is, after passing through an fθ lens configuring a post-deflection optical system, separated into beams for each of the color components and irradiated on a photoconductive drum for each of the color components.
There is also proposed a color image forming apparatus including an optical beam scanning apparatus that forms plural scanning lines by separating one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, imaging the luminous fluxes by a post-deflection optical system. This optical beam scanning apparatus determines timing for drawing an image in a main scanning direction on photoconductive drums by detecting timing of beams irradiated on the respective photoconductive drums with use of a horizontal synchronizing sensor. Several methods are known as a method of guiding luminous fluxes to the horizontal synchronizing sensor.
For example, according to a technique proposed in JP-A-11-218991, a horizontal synchronizing sensor provided in an optical path of a beam for black can detect only the beam for black and, without detecting beams for other colors, draw an image on photoconductive drums after predetermined time subsequent to the detection of the beam for black. According to a technique proposed in JP-A-63-274263, it is possible to guide beams of plural scanning lines to one horizontal synchronizing sensor through an optical fiber and then, set a signal time width of a horizontal synchronizing signal for each of the scanning lines. According to a technique proposed in JP-A-63-273814, it is possible to guide beams of plural scanning lines to one horizontal synchronizing sensor through an optical fiber or guide beams of scanning lines to one horizontal synchronizing sensor by changing an angle of a reflection mirror that guides the beams to the horizontal synchronizing sensor.
In the technique proposed in JP-A-11-218991, theoretically, it is possible to adjust phases of laser beams of the respective colors by detecting the beam for black. However, in a situation in which there is an error in accuracy of angles of deflection surfaces of a deflecting device and an error can occur in rotating speed of the deflecting device, it is difficult to appropriately adjust phases of laser beams of the respective colors by detecting only the beam for black. In the technique proposed in JP-A-63-274263, it is necessary to make plural beams incident on the optical fiber. In order to make the plural beams sufficiently incident on the optical fiber, it is necessary to provide an adjusting mechanism for adjusting a position of the optical fiber. In the technique proposed in JP-A-63-274263, even if the plural beams are made incident on the optical fiber, it is impossible to discriminate on which of deflecting surfaces the beams are deflected.
Moreover, in the technique proposed in JP-A-63-273814, besides using the optical fiber, it is possible to adjust phases of laser beams of the respective colors by changing an angle of the reflection mirror. However, it is difficult to sufficiently adjust the phases of the laser beams of the respective colors.
In this way, in a color image forming apparatus including an optical beam scanning apparatus that separates one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, images the luminous fluxes by a post-deflection optical system, it is impossible to appropriately adjust phases of laser beams of respective colors. As a result, not only color drift occurs but also distortion occurs in an image of a single color.